1. Field of This Invention
This invention is related to heat exchange means for transferring heat indirectly from a hot gas stream flowing across finned tubing to a cold liquid stream flowing through the tubing.
2. Description of the Prior Art
Heat exchangers for transferring heat between a liquid stream and gas stream are commonly fabricated using externally-finned tubing. An example is a gas-fired hydronic boiler. One approach for achieving a compact high-effectiveness configuration is to wrap the tubing in the form of a coil in which the liquid stream flows inside the tubes and the gas stream flows across the tubes and between adjacent rows of the coil.
An important problem with this type of configuration is that a significant fraction of the external surface area of the tubes is ineffective in transferring heat. This is because a high heat transfer coefficient on the hot-gas side of the heat exchanger is achieved only in the high-velocity region between adjacent rows of the coil through which the gas stream flows. A second problem is directly related to the difficulty in maintaining a uniform spacing between tubes in wrapping the coil. The resulting variation in gap size causes a nonuniform distribution of flow on the hot-gas side. This problem is especially acute when the fin height is small in comparison to the tube diameter. A nonuniform flow distribution is undesirable because it reduces the overall heat-transfer effectiveness achievable with a given coil geometry.
A method of alleviating these problems is to wrap a sheet metal strap around the coil between adjacent tube rows such that it contacts the fins of the adjacent rows. When wrapped in this manner the strap acts as a baffle forcing the hot-gas stream to flow at a high velocity over a greater portion of the tube surface thereby increasing the heat-transfer effectiveness. With the baffle, the gas is forced to flow through the space between the fins. Since the fin diameter and spacing are considerably more uniform than the gap between tube rows, the baffle also significantly reduces the degree of flow maldistribution.